The human spine functions through a complex interaction of several parts of the anatomy. FIGS. 1 and 2 (FIG. 2 being the cross-section A-A of FIG. 1) illustrate a segment of the spine 4, with vertebra 5. The vertebra 5 includes the vertebral body 6, the spinous process 8, transverse process 10, pedicle 12, and laminae 14. A functional spine, comprising several vertebra 5, typically subcategorized as being part of the cervical, thoracic, lumbar, sacral, and coccygeal regions as known, provides support to the head, neck, trunk, transfers weight to lower limbs, protects the spinal cord 20, from which peripheral nerves 32 extend, and maintains the body in an upright position while sitting or standing.
Also illustrated in FIGS. 1 and 2, the spinal segment 4 includes intervertebral discs 20 that separate adjacent vertebra 5. The intervertebral discs 20 provide motion, load bearing and cushioning between adjacent vertebrae 5. Intervertebral discs 20 are the largest avascular structure in the body, relying on diffusion for nutrition. The diffusion of nutrients is aided by the compression cycles that the intervertebral discs 20 undergo during the course of normal movement, which drives out waste products and cycles fluids. Lying down and resting reduces the load on the intervertebral discs 20 allowing nutrients to diffuse into the intervertebral discs 20.
Also illustrated in FIGS. 1 and 2, the spinal segment includes spinal facet joints 16. Spinal facet joints 16 join the adjacent vertebrae 6. The spinal facet joints 16 are synovial joints that function much like those of the fingers. Together with the intervertebral disc 20, the spinal facet joints 16 function to provide proper motion and stability to a spinal segment 4. Thus, each spinal segment 4 includes three joints: the intervertebral disc 20 in the anterior aspect of the spinal segment 4 and the two spinal facet joints 16 in the posterior aspect of the spinal segment 4.
For the spinal segment 4 to be healthy, each of the intervertebral disc 20 and the spinal facet joints 16 must be healthy. To remain healthy these joints require motion. The intervertebral disc 20 and the spinal facet joints 16 function together to provide both quality and quantity of motion. The quality of the motion is a exhibited by the non-linear energy storage (force-deflection, torque-rotation) behavior of the spinal segment 4. The quantity of motion is the range of segmental rotation and translation.
Back pain due to diseased, damaged, and/or degraded intervertebral discs 20 and/or spinal facet joints 16 is a significant health problem in the United States and globally. A non-exhaustive and non-limiting illustration of examples of diseased and/or damaged intervertebral discs is shown in FIG. 3. While a healthy intervertebral disc 20 is illustrated at the top of the spine segment 18, diseased and/or damaged discs are also illustrated. The diseased and/or damaged discs include a degenerated disc 22, a bulging disc 24, a herniated disc 25, a thinning disc 26, discs indicating symptoms of degeneration with osteophyte formation 28, as well as hypertrophic spinal facets 29.
A degenerating spinal segment 18 is believed to be the product of adverse changes to its biochemistry and biomechanics. These adverse changes create a degenerative cascade affecting the quality and/or quantity of motion and may ultimately lead to pain. For example, as the health of a spinal segment 18 degenerates and/or changes, the space through which the spinal cord 30 and peripheral nerves 32 (FIGS. 1 and 2) pass can become constricted and thereby impinge a nerve, causing pain. For example, the spinal cord 30 or peripheral nerves 32 may be contacted by a bulging disc 24 or herniated disc 25 or hypertrophic spinal facet 29 as illustrated in FIG. 3. As another example, a change in the spinal segment 18, such as by a thinning disc 26 may alter the way in which the disc functions, such that the disc and spinal facets may not provide the stability or motion required to reduce muscle, ligament, and tendon strain. In other words, the muscular system is required to compensate for the structural deficiency and/or instability of the diseased spinal segment 18, resulting in muscle fatigue, tissue strain, and hypertrophy of the spinal facets, further causing back pain. The pain this causes often leads patients to limit the pain-causing motion. However, this limiting of motion, while offering temporary relief, may result in longer-term harm because the lack of motion limits the ability of the disc to expel waste and obtain nutrients as discussed above.
In many instances of degenerative disc disease, fusion of the vertebrae is the standard of care for surgical treatment, illustrated in FIG. 4. In the U.S. alone, approximately 349,000 spinal fusions are performed each year at an estimated cost of $20.2 billion. The number of lower back, or lumbar, fusions performed in the U.S. is expected to grow to approximately 5 million annually by the year 2030 as the population ages, an increase of 2,200%.
Spinal fusion aims to limit the movement of the vertebra that are unstable or causing a patient pain and/or other symptoms. Spinal fusion typically involves the removal of a diseased disc 50, illustrated in outline in FIG. 4. The removed disc 50 is replaced by one or more fusion cages 52, which are filled or surrounded by autograft bone that typically is harvested by excising one or more spinal facet joints 57. Vertebral bodies 51 adjacent the removed disc 50 are stabilized with one or more posterior supports 58 that are fixedly connected to the vertebral bodies 51 with the use of pedicle screws 54 that are screwed—such as by use of a bolt-style head 56 to turn the pedicle screw 54—into a hole drilled into the pedicle 12 of the vertebral bodies 51.
Fusion, however, often fails to provide adequate or sufficient long-term relief in about one-half of the treatments, resulting in low patient satisfaction. Further, fusion, by definition, restricts the overall motion of the treated functional spine unit, imposing increased stresses and limiting range of motion on those portions of the spinal segment adjacent to the fused vertebral bodies 51. Fusion of a spinal segment has been indicated as a potential cause of degeneration to segments adjacent to the fusion. The adjacent spinal facet joints 57 and adjacent discs 59 often have to bear a greater load as a result of the fusion than would typically be the case, leading to possible overloading and, in turn, degeneration. Thus, surgical fusion often provides short-term relief, but possibly greater long-term spinal degradation than would otherwise have occurred.
Thus, a challenge to alleviating the back pain associated with various ailments is to find an intervertebral disc prosthesis that provides sufficient freedom of movement to at least reduce the risk to the functional health of the adjacent spinal segments, and/or facet joints, and/or discs that are otherwise compromised or have their functional health degraded by spinal fusion, and, more preferably, maintain the functional health of the adjacent spinal segments and/or facet joints and/or discs. Further, an intervertebral prosthesis optionally provides sufficient stability to the diseased segment to alleviate pain and/or other symptoms.
A further challenge is simply the complex, multi-dimensional nature of movement associated with a functional spine unit. Illustrated in FIG. 5 are the varying axes around which a functional spine unit moves. For example, a vertebra 5 is illustrated with an X-axis 60, around which a forward bending motion, or flexion, 61 in the anterior direction occurs. Flexion 61 is the motion that occurs when a person bends forward, for example. A rearward bending motion, or extension, 62 is also illustrated. The Y-axis 63 is the axis around which lateral extension, or bending, 64, left and right, occurs. The Z-axis 65 is the axis around which axial rotation 66, left and right, occurs. Spinal fusion, as discussed above, limits or prevents flexion 61-extension 62, but also limits or prevents motion in lateral extension, or bending, 64 and axial rotation 66. Thus, an improved alternative remedy to fusion preferably allows for movement with improved stability around each of the three axes, 60, 63, and 65.
Another difficulty associated with the complex motion of the spine is that the center-of-rotation for movement around each of the X-axis 60, Y-axis 63, and Z-axis 65 differs for each axis. This is illustrated in FIG. 6, in which the center-of-rotation for the flexion 61-extension 62 motion around the X-axis 60 is located at flexion-extension center-of-rotation 70. The center-of-rotation for the lateral extension, or bending, 64 motion around the Y-axis 63 is located at lateral extension, or bending, center-of-rotation 73. The center-of-rotation for the axial rotation 66 around the Z-axis 65 is located at axial rotation center-of-rotation 75. For more complex motion patterns (e.g., combined flexion, lateral extension/bending, etc.) a two-dimensional representation of the center-of-rotation is inadequate, but the three-dimensional equivalent called the helical axis of motion, or instantaneous screw axis can be employed. Intervertebral disc prostheses that force rotation of a spinal segment around any axis other than the natural helical axis impose additional stresses on the tissue structures at both the diseased spinal segments and the adjacent spinal segments. Compounding the issue for the centers-of-rotation is that they actually change location during the movement, i.e., the location of the centers-of-rotation are instantaneous, which is sometimes referred to as the helical axis. Thus, a preferable remedy to spinal problems would account for the helical axis throughout the range of motion. Stated differently, a preferable intervertebral disc prosthesis would allow the diseased spinal segment and adjacent spinal segments to undergo motion approximate that of the natural helical axis through the range of motions.
Many previous efforts have been made to solve at least some of the problems associated with spinal fusion, but with varying degrees of success. For example, U.S. Patent Publication No. 2008/0195213 filed on Feb. 11, 2008 to several of the present inventors, discloses an intervertebral disc prosthesis that provides for motion in two directions, typically flexion-extension and lateral extension/bending, but not for axial rotation. (U.S. Patent Publication No. 2008/0195213 is incorporated herein in its entirety for all purposes by this reference.)
Thus, there exists a need for an intervertebral disc prosthesis that provides for flexion-extension, lateral extension/bending, and axial rotation.
Further, there exists a need for an intervertebral spinal prosthesis that reduces the stress on a diseased and/or damaged spinal segment without overloading the adjacent discs and vertebrae that could initiate progressive degeneration or diseases in the adjacent discs and vertebrae.
A need also exists for a spinal implant that provides for proper force-deflection behavior of the spinal implant (kinetics)—as noted above in the discussion of FIG. 5—preferably to approximate those of a normal, functional spine unit to relieve the load and strain on the adjacent intervertebral discs, to protect the spinal facet joints, to reduce the risk of damage to segments of the spine adjacent to the diseased segment, to reduce muscle fatigue and reduce and/or eliminate subsequent pain.
A need also exists for a spinal implant that exhibits kinematics—such as the limits of the ranges-of-motion and the centers-of-rotation noted above in the discussion of FIG. 6—that, preferably, are maintained near those of a functional spine unit to maintain an effective range of motion for the intervertebral discs, spinal facet joints, muscles, ligaments, and the tendons around the spine and to reduce the amount of neural element strain, e.g., the strain on the spinal cord and/or other parts of the nervous system.